1. Field of the Invention
The present invention relates to a control device for a continuously variable transmission installed in a vehicle.
2. Description of the Related Art
A belt type continuously variable transmission (CVT) incorporated into a power transmission system of a vehicle comprises a primary pulley provided on an input shaft, a secondary pulley provided on an output shaft, and a drive belt wound around the pulleys, and controls a gear ratio continuously by varying the winding diameter of the drive belt relative to the pulleys. The primary pulley and secondary pulley each comprise a fixed sheave and a movable sheave facing the fixed sheave, and by moving the movable sheave axially, the winding diameter and tension of the drive belt can be controlled.
For example, to control the tension of the drive belt using the secondary pulley, a target secondary pressure is calculated on the basis of a target gear ratio and an input torque, and a secondary pressure that has been adjusted toward the target value is supplied to the secondary pulley. To control the winding diameter of the drive belt using the primary pulley, a target gear ratio is set on the basis of a throttle opening, a vehicle speed, and so on, and a hydraulic ratio between the primary pressure and secondary pressure that corresponds to the target gear ratio is set. A target primary pressure is then set on the basis of the hydraulic ratio and the aforementioned target secondary pressure, whereupon a primary pressure that has been adjusted to the target value is supplied toward the primary pulley.
To adjust the primary pressure and secondary pressure in this manner, a hydraulic control circuit is provided with a primary pressure control valve for adjusting the primary pressure and a secondary pressure control valve for adjusting the secondary pressure, and control signals based on running conditions are output to these control valves from an electronic control unit. In particular, a control signal that has been subjected to feedback control on the basis of a deviation between the target gear ratio and an actual gear ratio is output to the primary pressure control valve for adjusting the primary pressure in order to cause the actual gear ratio to converge with the target gear ratio.
An integral correction value calculated through integration processing is typically used during this feedback control. However, the integral correction value accumulates when the target gear ratio and actual gear ratio do not match, and as a result the responsiveness of speed change control may deteriorate. To avoid this situation, a control device that prohibits integration processing when the target gear ratio is set on an overdrive side below a lower limit gear ratio and when the target gear ratio is set a low side higher than an upper limit gear ratio has been proposed (see Japanese Unexamined Patent Application Publication H3-249464, for example). Further, a control device that avoids unnecessary accumulation of the integral correction value by setting an upper limit on the integral correction value accumulation amount and resets the integral correction value when the deviation between the target gear ratio and actual gear ratio shifts to a reverse symbol has also been proposed (see Japanese Patent Publication No. 3596447, for example).
The primary pressure control valve and secondary pressure control valve are controlled on the basis of control signals from the electronic control unit, but a failsafe function is incorporated into the control circuit so that a minimum running performance can be secured while maintaining running safety even when a state of failure occurs due to disconnection of the control valves or the like such that control cannot be performed. If the supply of oil pressure from the primary pressure control valve and secondary pressure control valve were to be halted during a failure, for example, slippage would occur in the drive belt and so on, making control of the vehicle impossible, and therefore a valve structure allowing output of the maximum primary pressure and secondary pressure during a failure is employed.
However, when a large secondary pressure is output from the secondary pressure control valve during a failure, the hydraulic ratio between the primary pressure and secondary pressure shifts, and as a result, the minimum gear ratio of the continuously variable gear ratio is raised to the low side. In other words, when the secondary pressure control valve enters a state of failure, excessive secondary pressure is output, and as a result, the speed change region in which control can be performed is limited to a predetermined region on the low side.
When the secondary pressure control valve enters a state of failure on the overdrive side, the target gear ratio and actual gear ratio diverge greatly such that when an attempt is made to bring the low side actual gear ratio close to the overdrive side target gear ratio, the aforementioned integral correction value accumulates unnecessarily on an upshift side. When a downshift command is output under these conditions, a downshift operation of the continuously variable transmission is essentially halted until the integral correction value that has accumulated on the upshift side is expended, and as a result, responsiveness during speed change control deteriorates. Furthermore, when a downshift is executed during an emergency stop while the integral correction value is in an accumulated state, the vehicle may stop before the downshift to a low state is complete, and therefore, due to a lack of drive power, it may be difficult to restart the vehicle.